Face sitting is, in my experience, one of the main differences between traditional BDSM Mistresses and fbbs who are dominant / sensual.
The traditional Mistresses years ago hardly ever discussed face sitting in public forums, and if they did (or do now on their own web sites) insist that they will be clothed - panties, tight slacks, shorts, etc. I think it was / is a fear of vice laws.
More than a few I saw over the years at some point insisted I kiss their ass, which in the heat of the moment, I did, but it was not face sitting. One Mistress I saw last 20 or more years ago, right before I entered the FBB session world, was Amazonian as per my role play preference of being a captured Tarzan for the Amazon Queen. I was a regular of hers and she was around my height, 6 ft. or so, in heels. Not a bodybuilder but a woman with wide shoulders, strong back & thighs (never had her scissor me but she would have been dangerous) and probably had a 44DD-29-39 figure... probably about 170 - 175 lbs. I was 225 or 230 lbs. and she was hard to handle. Again, never advertised it, but by the second or third session she had me on the floor as if it was a non-competitive wrestling match and her talking trash. To my shock she sat on my face - completely naked. She was facing my feet, still talking trash.
No, I did NOT want to kiss her asshole or smell it, but I did get into kissing each cheek and I moved from one side to the other. She caught on and helped a bit, and must of liked it because that was the first session I ever worshipped her clit.
With FBBs, face sitting seems to be on the table if they do sensual sessions, and are usually nude or wear a thong. That's what I like about our gals who are sensual - they are used to showing off their bodies and, if they are really into you, will get more sensual than you would ever expect.